Recent developments in high rate cathodic sputtering, particularly those obtained using magnetic fields which enhance the sputtering, presently provide a major contribution to the coating field. This is particularly true in that situation where relatively large surface areas of substrate are being coated by the use of fairly large sputtering targets. A problem in the commercial operation of a sputtering target is the desirability of an essentially continuous production or manufacturing. This, of course, gives rise to a fairly fast consumption of the sputtering target. As the target consumption proceeds, the point may be reached where either piercing the target sputtering or eroding of the underlying conductive target supporting material will occur. It is a continuing concern to be able to avoid such erosion of the underlying conductive material. In the case of a sputtering target that is backed by direct water cooling, there is that great danger of piercing the target and water passing through the target into the sputtering chamber.
High rate sputtering systems presently used are designed to insure target cooling, which is necessary because of the relatively high power used, and these cooling systems have become one of two basic designs. One is termed the direct cooling arrangement where the liquid coolant is circulated directly along the back of the sputtering target, in which case the target piercing causes some major damage within the vacuum chamber by flooding it with coolant. The direct cooling arrangement as described above is used where the sputtering material or target is bonded to a suitable intermediate support. Generally, in the case of a cylindrical sputtering system where the target is in the form of a cylinder, the underlying target support would be a conductive copper cylinder or possibly an aluminum cylinder. In this instance, the piercing of the target or the erosion of the target completely through at some point would result in the sputtering of the underlying copper or aluminum substrate. Again, this is undesirable from the point of view that it would require replacement or costly repair to the underlying substrate. In this instance, the cooling would be applied to the interior of the substrate.
At the present time, the exact instant of target piercing is unforseeable and, to be on the cautious side, usually the time of sputtering is logged and when the estimated depletion time is approaching, the system is shut down and the target replaced. This shutting down of the system when there may be several days or, at the least, several hours of safe sputtering is a costly and wasteful procedure. In normal operation, however, it is better to err on the side of caution rather than try to run the sputtering operation for too long a period and harm the underlying substrate or pierce the target to the extent of letting coolant to enter the sputtering chamber.
A system for determining target piercing has been the subject of a recent U.S. Pat. No. 4,374,722, issued Feb. 22, 1983. This patent teaches the positioning of a gas chamber behind the sputtering target and under sufficient pressure such that the gas, when the target becomes pierced, will pass into the sputtering chamber. The presence of the gas in the sputtering chamber is then detected and the system shut down so that the target may be replaced.
The above patent discloses the use of the principle of the invention upon which the patent is based, to both cylindrical targets as shown, for example, in FIG. 1, or planar targets, for example, as shown in FIG. 7. In both instances, the target is of the magnetically enhanced type which produces a more efficient sputtering system. The cylindrical targets of the above-cited patent are for the purpose of sputtering on cylindrical substrates or substrates in surrounding relationship thereto.
Another recently issued U.S. Pat. No. 4,356,073 discloses a magnetron cathode sputtering apparatus in which the cathode or target is in the form of an elongated, cylindrical tube having a layer of the coating material to be sputtered applied to the outer surface thereof. Magnetic means is mounted within the tube and includes at least one row of permanent U-shaped magnets extending lengthwise of the tube. This tube is horizontally arranged and the magnets are positioned in the lower, inside portion of the tube. The substrate to be sputtered is positioned beneath the tube and may be moved relative thereto to provide sputtering over a relatively long substrate, with the substrate's width being somewhat less than the length of the cylinder. The significant configuration of this patent is the ability to rotate the cylinder about its horizontal axis while maintaining the magnets in a constant downward orientation to provide a means for sputtering the target in a uniform manner about its entire cylindrical surface. Obviously, this provides an economical system, since the target may remain in the chamber for an extended period before requiring replacement. Both patents cited above teach the necessity of providing a cooling system for the interior in the case of a cylinder or reverse side of the planar cathode. In both patents, it is suggested that the cooling be effected by the use of circulating water in the cathode.
With the foregoing in view, it is an object of the present invention to provide a system for determining when the target material has become depleted to the extent that it requires the target to be replaced in the sputtering system.
It is a further object of this invention to provide a system for determining the impending depletion of the target material, whether the target material is a coating on a cathode support, or whether it is actually a part of the material of the cathode itself.
It is a further object of this invention to provide a system which will permit the operator to be alerted to the impending depletion at a point in time which will be sufficient to permit the timely shutdown of the sputtering system to avoid any possibility of significant erosion of the cathode having the target material thereon or to alert the operator to the depletion of the cathode itself when it is formed of the target material.
Other and further objects will be apparent from the following description taken in conjunction with the annexed sheets of drawings.